Ataxia-telangiectasia (A-T) is a rare, autosomal recessive disorder that causes progressive neurodegeneration, immunodeficiency, and predisposition to malignancy. Studies of this disease have been hampered by its rarity, so that even the largest medical centers have followed fewer than a dozen patients. Four years ago, we established the A-T Clinical Center at Johns Hopkins as a joint venture between the Pediatric Clinical Research Unit and a private foundation, the A-T Children's Project. We have since seen more than 150 patients from all parts of the United States, as well as Canada, Mexico, South and Central America, Europe and Africa. We have used this large patient base to more carefully define the phenotype of A-T. We have documented that the diagnosis of A-T is usually made years after the onset of ataxia because most clinicians do not suspect the diagnosis until telangiectasia appear. We have developed a scoring scale for the clinical assessment of neurologic disease. This has been useful for recognizing A-T variants, and will be indispensable for monitoring future therapeutic trials. We have identified extraordinarily high frequencies of oligoclonal gammopathy and hypergammaglobulinemia. These were previously unrecognized and may be sentinel features of premature senescence of the immune system in patients with A-T. Studies in progress will help to define the etiology of chronic/recurrent pulmonary infections, and define the relationship between genotype and phenotype.